Any ophthalmic lens intended to be held in a frame involves a prescription. The ophthalmic prescription can include a positive or negative power prescription as well as an astigmatism prescription. These prescriptions correspond to corrections enabling the wearer of the lenses to correct defects of his vision. A lens is fitted in the frame in accordance with the prescription and with the position of the wearer's eyes relative to the frame.
In the simplest cases, the prescription is nothing more than a power prescription. The lens is said to be unifocal and has a rotational symmetry. It is fitted in a simple manner in the frame so that the principal viewing direction of the wearer coincides with the axis of symmetry of the lens.
For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far vision power value and an addition (or power progression) representing the power increment between far vision and near vision; this comes down to a far vision power prescription and a near vision power prescription. Lenses suitable for presbyopic wearers are progressive multifocal lenses; these lenses are described for example in FR-A-2 699 294, U.S. Pat. No. 5,270,745 or U.S. Pat. No. 5,272,495, FR-A-2 683 642, FR-A-2 699 294 or also FR-A-2 704 327. Progressive multifocal ophthalmic lenses include a far vision zone, a near vision zone and an intermediate-vision zone, a principal progression meridian crossing these three zones. They are generally determined by optimization, based on a certain number of constraints imposed on the different characteristics of the lens. These lenses are all-purpose lenses in that they are adapted to the different needs of the wearer at the time.
For young presbyopics, lenses have been proposed which do not have a far vision zone with a reference point, unlike standard progressive multifocal lenses; these lenses are described in FR-A-2 588 973. These lenses are prescribed only according to with the power required by the wearer in near vision, regardless of the power required by the wearer in far vision. The lens has a central part which has an additional spherical power offering the wearer satisfactory near vision. It also has a slight decrease in power in the upper part, which ensures the wearer clear vision even beyond the normal near vision field. Finally, the lens has a point with a power value equal to the nominal near vision power, a higher-power zone in the lower part of the lens and a lower-power zone in the upper part of the lens.
FR-A-2 769 997 proposes a lens which, compared with a standard progressive multifocal lens, has a stabilized and larger near vision zone, a significant increase in the field widths in near vision and intermediate vision, as well as a reduction in aberrations and in particular in astigmatism. It ensures a suitable correction for distances between 40 and 80 cm and, in most cases, for distances between 40 cm and 2 m. This lens is actually a near vision—intermediate-vision mid-distance lens, favouring near vision while ensuring clear vision beyond the normal near vision field. On the other hand, no far vision is available. This lens proves particularly well suited to computer work. It is prescribed for young presbyopics, solely according to the prescription for near vision. The rear face of the lens is machined in order to ensure a near vision power matching the prescription, without taking account of the far vision prescription. Two front faces are sufficient to meet all of the wearer's needs.
FR-A-2 769 999 proposes a progressive multifocal ophthalmic lens having improved comfort of vision with a monotone sphere variation as a function of the angle on a circle of radius 20 mm centred on the geometrical centre of the lens on either side of the meridian. This lens ensures unimpeded far vision covering an angular sector originating at the geometrical centre of the lens and having an angle at the centre greater than 150°.
Multifocal lenses, whether they are progressive or intended purely for near vision, can include a complex multifocal face (i.e. allowing no axis of rotation, typically a surface having a power progression), for example the face away from the person wearing the glasses, and a spherical or toric face, called the prescription face. This spherical or toric face allows the lens to be adapted to the user's ametropia, so that a multifocal lens is generally defined only by its complex surface. For a given product different complex faces are defined in accordance with the addition and the base (or mean far vision sphere). Starting from semi-finished lenses, of which only the multifocal face is formed, it is possible to prepare lenses suited to each wearer, by simple machining of a spherical or toric prescription face.
Independently of the power prescription, a wearer may be given an astigmatism prescription. Such a prescription is produced by the ophthalmologist, in the case of far vision, in the form of a pair formed by an axis value (in degrees) and an amplitude value (in diopters). On a surface, the amplitude value represents the difference 1/R1−1/R2 between the principal curvatures; the axis value represents the orientation, relative to a reference axis and in a conventional direction of rotation, of the maximum curvature 1/R1.
In prescription terms, the amplitude value represents the difference between the minimum and maximum powers in a given direction and the axis represents the orientation of the maximum power. The term astigmatism is used for the pair (amplitude, angle); this term is also sometimes used, although this is linguistically incorrect, to designate the amplitude of the astigmatism. The context allows a person skilled in the art to understand which meaning is intended.